Method and device for optically measuring distance

ABSTRACT

The invention relates to a method for optical distance measurement, in which a transmitter of a transmission branch ( 14 ), which branch is integrated with a measuring device ( 10 ), transmits a modulated measurement beam ( 16, 36 ) in the direction of a target object ( 20 ), and the measurement beam ( 17, 44 ) reflected or scattered by the target object ( 20 ) is detected by a measurement receiver ( 54 ) integrated with the measuring device ( 10 ) and is transformed by a frequency mixing process into a low-frequency range.  
     According to the invention, for detection and frequency transformation of the returning measurement signal ( 17, 44 ), a measuring diode ( 62 ) whose cathode voltage and anode voltage are modulated with high frequency to generate a mixing signal is used.  
     An apparatus ( 10 ) is also proposed for performing the method of the invention.

The present invention is based on a method for optical distancemeasurement as generically defined by the preamble to claim 1 and on anapparatus for performing such a method as generically defined by thepreamble to claim 4.

PRIOR ART

Distance measuring devices and in particular optoelectronic distancemeasuring devices have long been known per se. These devices transmit amodulated measurement beam, for instance a beam of light or a laserbeam, which is aimed at a desired target object whose distance from thedevice is to be ascertained. The returning measurement signal reflectedor scattered by the target object aimed at is detected again at least inpart by a sensor in the device and used to ascertain the distancesought.

In the known devices of this type, a distinction is made betweenso-called phase measurement methods and pure transit time measurementmethods for determining a sought distance from the target object. In thetransit time measurement method, a pulse of light of the briefestpossible pulse duration is transmitted by the measuring device, and thenits transit time to the target object and back into the measuring deviceis ascertained. With the known value of the speed of light, the distanceof the measuring device from the target object can be calculated fromthe transit time.

In the phase measurement method, conversely, the variation in the phaseof the measurement signal is utilized as a function of the distancetravelled to determine the distance between the measuring device and thetarget object. From the magnitude of the phase displacement impressed onthe returning measurement signal, compared to the phase of thetransmitted measurement signal, the distance travelled by themeasurement signal can be determined, and thus the distance between themeasuring device and the target object can also be determined.

The range of application of the distance measuring devices generallycovers distances in the range from a few centimeters to several hundredmeters. Meanwhile, such measuring devices are commercially sold incompact versions and make it simple for the commercial or private userto operate them, even in handheld form.

To attain high measurement precision with these devices, it is known toselect and use as high as possible a modulation frequency. However,since nonambiguity of the phase measurement exists only for a phaseangle between 0 and 360°, it is usual and also known for instance fromGerman Patent Disclosure DE 43 03 804 A1 to alternate a high modulationfrequency of the transmitted light beam with at least one further,substantially lower modulation frequency of the transmitted light beam,in order to attain a measurement range that goes beyond the phase anglerange of 0 to 360° for the high modulation frequency.

It is also known, for more-precise ascertainment of a phase differencebetween the transmitted and the received measurement signal, totransform the signal to be analyzed to a markedly lower frequency, forinstance by a frequency mixing process. This mixing process yields alow-frequency measurement signal which continues to be a carrier of thefundamental information, namely the phase displacement between thetransmitted and the received signal, but because of its markedly reducedfrequency is also substantially simpler to process further and can beevaluated more precisely.

To attain “downward mixing” of the measurement frequency, it is known tomix the transmission and reception signals with a signal whose frequencyis displaced only far enough from the measurement frequency that anoutcome of mixing is in the low-frequency range. In this low-frequencyrange, it is then no problem to measure the desired phase by means of asuitable switching device. Advantageously, the diode that detects thereturning measurement signal can be used for this frequency mixingprocess.

From German Patent Disclosure DE 37 43 678 A1, an optical backscatteringmeasuring device is known which has an optical transmitter whosetransmission power can be modulated via an oscillator with a varyingfrequency. The transmission beam of the backscattering measuring deviceof DE 37 43 678 A1 is carried via a beam splitter into the opticalwaveguide to be examined. The portions of the transmitted beam that arebackscattered by the optical waveguide are carried via the beam splitterto an optical receiver, embodied as a photodiode, of the opticalbackscattering measuring device. For ascertaining the location andintensity of the backscattering, in the backscattering measuring deviceof DE 37 43 678 A1, a mixed signal is formed from a signal that isproportional to the optical backscattering power and a modulationvoltage that has the oscillator frequency. The expense for the opticalreceiver is reduced by providing that the photodiode is an avalanchephotodiode, whose bias voltage is a direct voltage modulated by themodulation voltage. The low-frequency mixed signal thus generated ispicked up at a parallel circuit, connected into the exciter circuit ofthe photodiode, that comprises an active resistor and a capacitor.

From European Patent Disclosure EP 0 932 835 B1, an apparatus forcalibrating distance measuring instruments is known that has atransmitter which emits a high-frequency-modulated optical radiation andwith it illuminates a measurement object. The apparatus of EP 0 932 835B1 furthermore comprises a measurement receiver, which detects theradiation reflected by the measurement object and converts it into anelectrical signal. From the transmitter beam path of the distancemeasuring instrument of EP 0 932 835 B1, some of thehigh-frequency-modulated transmitter radiation is permanentlyout-coupled and delivered, via an internal reference path serving as acalibration path, directly to a reference receiver, such as a PIN diode.This diode is connected to a frequency mixer. The frequency mixer is inturn connected directly to an avalanche photodiode, used as ameasurement receiver for the measurement beam. A high-frequencyelectrical signal is coupled as a mixer frequency into this connection.This mixer frequency is mixed, via the frequency mixer, with thehigh-frequency modulation signal of the reference beam received from thereference receiver, and the result is a low-frequency calibrationsignal. On the other hand, the mixer frequency is mixed with thehigh-frequency modulation signal of the measurement beam received by theavalanche photodiode, and a result is a low-frequency measurementsignal. Thus in the apparatus of EP 0 932 835 B1, the avalanchephotodiode is a so-called direct mixer. The low-frequency calibrationsignal and the likewise low-frequency measurement signal are thendelivered in a known manner to where the phase measurement is done.

ADVANTAGES OF THE INVENTION

The apparatus of the invention for optical distance measurement asdefined by claim 4 has a transmission branch, with at least onetransmitter for transmitting modulated measurement beam in the directionof a target object. Advantageously, this transmitter is embodied as alaser diode, so that by modulation of the energy supply delivered to thelaser diode, the desired high-frequency modulation can be impresseddirectly on the optical signal. The apparatus of the invention moreoverhas a reception branch, and the measuring receiver of the receptionbranch is embodied as a direct-mixing avalanche photodiode. Thisphotodiode converts the incident optical signal into a correspondingelectrical signal of the same frequency.

As a result of the modulation of the gain of the photodiode, and inparticular because of the nonlinearity of the gain that determines thephotoelectric current of the diode, the electrical output signal of thephotodiode also has a signal whose frequency is defined by thedifference between the frequency of the modulation of the incidentoptical measurement signal and the modulation frequency of the gain ofthe photodiode. In this kind of direct mixture of the measurementsignal, the DC blocking voltage applied to the cathode of the diode,typically in the range from 50 V to 500 V, has a frequency-modulatedsmall-signal voltage superimposed on it. This modulating small-signalvoltage is typically sinusoidal but can have other frequency coursesinstead. Because of this modulation, thus along with the blockingvoltage, the gain factor of a diode is also modulated as a function offrequency.

Advantageously, in the apparatus of the invention for optical distancemeasurement, not only the cathode bias voltage U_(K) of the avalanchediode (U_(K)=U₀+U_(K)(t)), a voltage U_(A)=u_(A)(t) applied to the anodeis modulated as well. In this way, common-mode interference, whichoccurs both in the cathode path and the anode path of the measurementreceiver, can be reduced or avoided entirely. This common-modeinterference is no longer mixed in to its full intensity, since for themixing in the diode, only the differential signal between the cathodeand the anode plays a major role. Thus it is possible for instance foradulterations of the measured value caused by corresponding common-modeinterference to be reduced markedly.

By the provisions recited in the dependent claims, advantageousrefinements of the apparatus defined by the independent claims and ofthe corresponding claimed method are possible.

Advantageously, the apparatus of the invention has a circuit arrangementwith which the avalanche photodiode, serving as a measurement receiver,can be acted upon on the anode side by a time-dependent anode voltageu_(A)(t), which is equivalent to the inverted signal of the modulatedcathode voltage u_(K)(t). In the apparatus of the invention, thus boththe anode and the cathode sides of the avalanche photodiode aremodulated by a voltage of the same amount but of inverted polarity. Forthe mixing of the frequency of the measurement signal with themodulation of the diode used as a frequency mixer, only thetime-dependent differential signal U_(D)(t) between the cathode and theanode of the diode plays a major role. For the differential signalU_(D)(t), the equation is accordingly:U _(D)(t)=u _(A)(t)−u _(K)(t)=u _(A)(t)−u _(K)(t)=2u _(A)(t).

Compared to pure cathode modulation, in the method of the invention onlyhalf the modulation amplitude is thus needed to generate a modulationsignal of desired intensity. Particularly in the higher frequencyranges, this is advantageous, because the driver output for generatingthe modulation frequencies can be reduced accordingly. Moreover, theunavoidable reradiation of the system is reduced to one-quarter of thevalue that would result if only cathode modulation of the measurementreceiver were done, since for electrically short antennas, thereradiation increases quadratically with the amplitude of thetransmission power.

In an advantageous embodiment of the apparatus of the invention, asingular modulator furnishes not only the modulating cathode voltageu_(K)(t) but also the anode voltage u_(A)(t). In this embodiment, it ispossible by simple means to generate the desired modulations at theavalanche photodiode.

The photodiode converts the incident optical signal into a correspondingelectrical signal of the same frequency. As a result of the modulationof the gain of the photodiode, the electrical output signal of the diodealso has a signal whose frequency is defined by the difference betweenthe frequency of modulation of the incident optical signal and themodulation frequency for the gain of the photodiode. In the apparatus ofthe invention, not only the cathode bias voltage but also the anode biasvoltage are modulated. In the process in the photodiode, thedifferential signal between the cathode and the anode voltages comes toplay a major role.

Further advantages of the apparatus and method according to theinvention will become apparent from the drawings and the associateddescription.

DRAWINGS

In the drawing, one exemplary embodiment of the apparatus of theinvention for optical distance measurement is shown and will beexplained in further detail in the ensuing description. The drawingfigures, their description, and the claims directed to the inventioninclude numerous characteristics in combination. One skilled in the artwill also considered these characteristics and the claims referring tothem individually and put them together to make further usefulcombinations and claims.

Shown are:

FIG. 1: An apparatus for optical distance measurement, in a simplified,schematic overall view.

FIG. 2: A circuit arrangement for operating a measurement receiver ofthe apparatus of the invention for optical distance measurement.

FIG. 1 schematically shows an optical distance measuring device 10 withits most important components, for the sake of describing itsfundamental structure. The device 10 for distance measurement has ahousing 12, in which both a transmission branch 14 for generating anoptical measurement signal 16 and a reception branch 18 for detectingthe measurement signal 17 returning from a target object 20 areembodied.

The transmission branch 14, along with a number of components nototherwise shown, in particular has a light source 22, which in theexemplary embodiment of FIG. 1 is embodied by a semiconductor laserdiode 24. It is equally possible to use other light sources in thetransmission branch 14 of the apparatus of the invention. The laserdiode 24 of the exemplary embodiment of FIG. 1 emits a laser beam in theform of a focused beam of light 26 that is visible to the human eye. Tothat end, the laser diode 24 is operated via a control unit 28, which bysuitable electronics generates a modulation of the electrical inputsignal 30 to the diode 24. The control unit 28 in turn receives therequired frequency signals for modulation of the laser diode from acontrol and evaluation unit 58 of the measuring device of the invention.In other exemplary embodiments, the control unit 28 may also be adirect, integral component of the control and evaluation unit 58.

The control and evaluation unit 58 includes a circuit arrangement 59,which among other elements has at least one quartz oscillator forfurnishing the required frequency signals. With these signals, of whichtypically a plurality, at different frequencies, are used during adistance measurement, the optical measurement signal is modulated in aknown manner. The fundamental structure of such an apparatus and thecorresponding method for generating different measurement frequenciescan be found for instance in German Patent DE 198 11 550 C2, so that atthis point this reference is merely referred to, and the contents ofthis reference are expressly incorporated herein, but will not beaddressed further within the context of the present description.

The intensity-modulated focused beam 26 of light emerging from thesemiconductor diode 24 passes through a first optical element 32, whichbrings about an improvement in the beam profile of the measurement beam.Such an optical element is an integral component of a modern laserdiode. The measurement beam 26 then passes through a collimator lens 34,which generates a virtually parallel beam 36 of light that istransmitted by the apparatus in the direction of a target object 20 tobe measured.

Also found in the transmission branch 14 of the apparatus of theinvention shown in FIG. 1 is a device 38 for generating a reference path40 that is internal to the device and with which an internal calibrationof the measuring device can be performed.

The measurement signal 16 is out-coupled from the housing 12 of theapparatus 10 through an optical window 42. For the actual measurementoperation, the apparatus 10 is aimed at the desired target object 20whose distance from the measuring device is to be ascertained. Byactuation of an operator control element, not further shown, the opticalwindow 42 can be opened, so that the measurement beam 36 strikes thetarget object 20. The signal 17, reflected or scattered by the desiredtarget object 20, passes to a certain extent through an entry window 46to return into the housing 12 of the apparatus 10 of the invention. Themeasurement beam 20 entering the face end 48 of the apparatus 10 throughthe entry window 46 forms a returning measurement beam 44, which issteered toward a receiving lens element 50. The receiving lens 50focuses the returning measurement beam 44 at the active face 52 of areceiving device 54.

The receiving device 54 of the apparatus of the invention has aphotodiode 62, which in a known manner converts the arriving lightsignal 17 into an electrical signal, which is then carried onward, viasuitable connecting means 56, to a control and evaluation unit 58 of theapparatus 10. From the returning optical signal 17 and particularly fromthe phase displacement impressed on the returning signal, in comparisonwith the originally transmitted signal 16, the control and evaluationunit 58 ascertains the distance sought between the apparatus 10 and thetarget object 20. The ascertained distance can be imparted to the userof the instrument, for instance through an optical display device 60.

The receiving device 54 that detects the returning measurement beam 44has an avalanche photodiode 62, which in the apparatus of the inventionis simultaneously used as an element for frequency transposition. If thebias voltage is such a photodiode, which voltage determines the gain ofthe electrical signal generated, is modulated, then this variation asalready noted is also expressed in the electrical output signal of thediode.

If optical measurement radiation at the modulation frequency f_(M)strikes the active face 52 of the avalanche diode, and if the gain ofthe diode is modulated by the application of an alternating voltageu_(K)(f_(M)+Δf), then in the electrical output signal of the diode, notonly the total frequency of these two signals but also the differentialfrequency Δf of the two modulation frequencies will be found. This factknown per se is utilized to transform the optical,high-frequency-modulated measurement signal into the range of lowfrequency (the range up to a few kHz), so that for further evaluation,only low-frequency, easily manipulated signals have to be processed. Inthe apparatus of the invention, the avalanche photodiode is operated asa mixer element in such a way that besides a high-frequency modulationof the cathode side (U_(K)=U₀+u_(K)(t)), the anode side of the diode isalso modulated (U_(A)=u_(A)(t)).

To that end, in the apparatus of the invention, for instance by themethod disclosed in German Patent DE 198 11 550 C2, a frequency(f_(M)+Δf) that is quite close to the optical modulation frequency f_(M)is generated. This frequency (f_(M)+Δf) is modulated upward on thecathode side in a modulator 64 of the supply voltage (bias voltage) U₀of the diode. While in known instruments only the cathode voltage U_(K)of the diode is modulated, in the apparatus of the invention the anodevoltage U_(A) of the diode is also modulated, with the inverted signalu_(A)(t)=−u_(K)(t) of the modulated small-signal voltage of the cathodeside. Via suitable connecting lines 65, the modulator 64 furnishes notonly the modulated cathode voltage u_(K)(t) but also the anode voltageU_(A)=u_(A)(t) for the diode. These two voltages should as much aspossible be of opposite polarity and have the same amplitude.

FIG. 2 shows a schematic circuit arrangement for modulation according tothe invention of the measurement receiver of a distance measuringdevice. The avalanche photodiode 62 of the receiving device 54 is struckby the measurement beam 44, reflected or scattered by the target object,which is transformed by the diode into an electrical signal. Accordingto the invention, the avalanche photodiode is modulated on the cathodeside with a modulation voltage u_(K)=u_(K)(t), while at the same timethe anode side of the photodiode is modulated with what in terms ofamount is a modulation voltage that is as much as possible the same butis inverted in its polarity, u_(A)=u_(A)(t)=−u_(K)(t). Between themodulator 64 that furnishes both the modulating cathode voltage u_(K)(t)and the anode voltage u_(A)(t), and the photodiode 62, there is arespective adaptation network 66 and 68, each, in the exemplaryembodiment of FIG. 2. These adaptation networks 66 and 68 are optionaland may possibly be necessary in order to attain acceptable amplitudesfor the modulation voltages u_(K) and u_(A), respectively, in the rangeof the requisite high frequencies, or to compensate for parasiticinterference effects on the modulation voltages.

Advantageously, the modulator 64 furnishes both modulation voltages. Tothat end, two signals of the same kind of amount can be generated with aphase displacement of 180° and fed to the corresponding electrodes ofthe diode.

On the output signal side of the photodiode 62 of the invention, thereare low-pass filters 70 and 72 in the receiving device 54. The low-passfilters 70 and 72 block the high-frequency portions of the signal, andin particular the high-frequency modulation frequency for the gain ofthe diode as well as the electrical output signal of the diode,corresponding to the optical signal, and thus make it possible for onlythe low-frequency mixed signal to be delivered for further evaluation.In the process, the low-pass filters 70 and 72 should have the smallestpossible impedance courses for the operating frequency. In a very simpleexemplary embodiment, these low-pass filters can be embodied by aresistor and a capacitor.

The receiving device 54 of the invention furthermore has a directcurrent voltage source 74, which is used as a DC blocking voltage sourcefor fixing or regulating the operating point of the photodiode. The DCblocking voltage source furnishes the voltage difference U₀, applied viathe diode, with which the photodiode is biased in the blockingdirection. Superimposed on this DC signal on both the cathode and theanode sides are the modulated small-signal voltages u_(K)(t) andu_(A)(t), respectively.

The low-frequency mixed signal (the mixed product of an electricalmodulation frequency at the electrodes of the photodiode and thefrequency of the modulation of the optical measurement signal) isdelivered to an amplifier element 76, which amplifies the mixed signal,which carries the phase information, in a desired way prior to furtherevaluation. The individual components of the apparatus are connected toone another in a suitable way via electrical connecting lines (65).

The amplitude of the mixed signal before amplification is determinedessentially by the amplitude of the incident optical signal and by thegain in the photodiode. For the mixing in the diode, it is essentiallythe time-dependent differential signal U_(D)(t) between the cathodevoltage U_(K) and the anode voltage U_(A) that is responsible. The mixedsignal of the diode, which is proportional to U_(D)(t) for the apparatusaccording to the invention accordingly becomes:U _(D)(t)=U _(A)(t)−U _(K)(t)=u _(A)(t)−u _(K)(t)=2u _(A)(t)=−2u_(K)(t).

With the modulation according to the invention on both the cathode andanode side of the frequency-mixing avalanche diode, accordingly onlyhalf the modulation amplitude is needed, compared to pure cathodemodulation. This is advantageous particularly in the range of relativelyhigh and extremely high measurement frequencies of the optical signal.Since the modulation frequency of the gain of the photodiode should beas close as possible to the (high) modulation frequency of the opticalmeasurement signal, such a system requires correspondingly high powerfor the electronic frequency drivers. With the apparatus of theinvention, because of the reduced demands in terms of the necessarymodulation amplitude, it is therefore possible to lower the driver poweraccordingly as well.

Furthermore, the reradiation of the detection system, which is a lossmechanism, is also reduced to one-quarter of the value that isestablished in conventional modulation on only the cathode side. Thereceiving device, which with adequate precision acts as an electricalshort antenna, has a reradiation which increases quadratically with theamplitude of modulation, so that halving the modulation amplitude makesa marked reduction in the reradiation possible.

Common-mode interference, which occurs in the same way in both the anodeand the cathode paths, is no longer mixed in as well in the method ofthe invention and thus cannot cause any adulteration of measured values.For this purpose it is necessary that the modulating cathode voltageu_(K)(t) and the anode voltage u_(A)(t) have as much as possible exactlythe opposite polarity and the same amplitudes. The better this conditionis met, the better the circuit functions, and the more cleanly cancommon-mode interference in the measuring system be eliminated.

With the method of the invention and the apparatus of the invention forperforming this method, it is possible to assure the mixing efficiency,in the frequency transformation in the low-frequency range, at highermodulation frequencies as well. Thus it becomes possible to use highermodulation frequencies for the optical measurement signal andaccordingly to attain greater measurement precision for a distancemeasuring device. Moreover, the method of the invention makes itpossible to reduce the interfering radiation emitted by the associatedmeasuring device and to achieve better suppression of interferencefactors to the receiving unit of the measuring device.

Neither the method of the invention and the apparatus of the inventionfor performing this method are limited to the described exemplaryembodiment shown in the drawings.

1. A method for optical distance measurement, in which at least onetransmission unit of a transmission branch (14) of a measuring device(10) transmits modulated measurement beam (16, 36) in the direction of atarget object (20), and the measurement beam (17, 44) returning from thetarget object (20) is detected in the measuring device (10) by at leastone measuring diode (62), present in a reception branch (18) of thedevice (10), and delivered to a control and evaluation unit (58) of themeasuring device, and the at least one measuring diode (62) of thereception branch (18) is also used as a frequency-mixing component fortransformation of a measurement signal to be evaluated, characterized inthat besides the cathode voltage U_(K)(U_(K)=U₀+u_(K)(t)) of themeasuring diode (62), an anode voltage U_(A) of the measuring diode (62)is also modulated (U_(A)=u_(A)(t)).
 2. The method for optical distancemeasurement of claim 1, characterized in that the anode voltage U_(A) ismodulated (U_(A)=u_(A)(t)=−u_(K)(t)) with the inverted, modulatedcathode voltage (−u_(K)(t)).
 3. The method for optical distancemeasurement of claim 1 or 2, characterized in that the modulated cathodevoltage u_(K)(t) and the modulated anode voltage u_(A)(t) is generatedby a common modulator (64).
 4. An apparatus for optical distancemeasurement having at least one transmitter (14) with at least onetransmitter (22, 24) for transmitting modulated measurement beam (16,36) in the direction of a target object (20), and having at least onereception branch (18) with at least one measurement receiver forreceiving the measurement beam (17, 44) returning from the target object(20), and the measurement receiver (54) is provided with a photodiode(62) acting as a frequency mixer element, and having a control andevaluation unit (58) for ascertaining the distance from the apparatus(10) to the target object (20), characterized in that a diode biasvoltage applied to the diode (62) is modulated on both the cathode andanode sides.
 5. The apparatus of claim 4, characterized in that theanode voltage u_(A)(t) that modulates the anode side is essentiallyequal to the inverted cathode voltage u_(K)(t) (u_(A)(t)=−u_(K)(t))modulated on the cathode side of the diode.
 6. The apparatus of claim 4or 5, characterized in that the apparatus has the modulator (64), withthe aid of which both the modulated cathode voltage u_(K) and themodulated anode voltage u_(A) can be generated.
 7. The apparatus ofclaim 6, characterized in that electrical connecting means (65) whichhave at least one adaptation network (66, 68) are provided between themodulator (64) for generating the modulated cathode voltage and anodevoltage and the diode (62) used as a mixer element.
 8. The apparatus ofone of claims 4 through 7, characterized in that the photodiode (62) isan avalanche photodiode.